1. Field of the Invention
The present invention generally relates to a television camera and, more particularly, to a digital processing color TV camera having digitized signal processing units.
2. Description of the Prior Art
A solid-state camera utilizing a solid-state imaging element which is compact and light-weight, and has a low power consumption and is highly reliable is today largely used as a color television camera not only in the television broadcasting industry, but also in homes. Particularly in the field of the television broadcasting industry, demands for a digital color camera are increasing because improvements in reliability, handling capacity and quality of images can readily be accomplished and also because it can readily be connected with other equipment which have been digitized.
One example of such prior art digital color cameras comprises a processing circuit for effecting black-balance, white-balance and gamma corrections, a Y color difference matrix circuit for providing a brightness signal and a color difference signal, low pass filters for limiting bands, delay lines for time adjustment and an encoder circuit for effecting a quadrature two-phase amplitude modulation of two color difference signals and combining them with the brightness signal.
Assuming that three primary colors of red, green and blue contained in a video signal are expressed by R, G and B, respectively, the brightness signal Y can be expressed by the following equation. EQU Y=0.30R+0.59G+0.11B
The two color difference signals, denoted I and Q signals, respectively, can be expressed by the following respective equations. EQU I=0.60R-0.28G-0.32B EQU Q=0.21R-0.52G+0.31B
An NTSC output signal which is a standard television system is generally processed in the following manner by a color encoder to combine R, G and B sync signals so as to thereby provide a single signal (NTSC) signal.
The two color difference signals, that is, the I and Q signals, emerging from the Y color difference circuit have their respective bands restricted according to the NTSC standards to 1.5 MHz and 0.5 MHz, respectively. Specifically, the I signal must have such frequency characteristics that (1) the amount of the I signal attenuated at 3 MHz is smaller than 2 dB and (2) the amount of the I signal attenuated at 6 MHz is greater than 20 dB. On the other hand, the Q signal must have such frequency characteristics that (1) the amount of the Q signal attenuated at 0.4 MHz is smaller than 2 dB, (2) the amount of the Q signal attenuated at 0.5 MHz is smaller than 6 dB and (3) the amount of the Q signal attenuated at 0.6 MHz is greater than 6 dB.
Thus, as compared with the I signal, the Q signal is subjected to a steep band restriction. Therefore, if the band restriction is carried out by the use of a low-pass filter of a simple construction having few stages, the phase characteristic tends to be deteriorated and ringing such as overshoot and undershoot tends to occur. Conversely, if a low-pass filter of a complicated construction having an increased number of stages is employed, a favorable phase characteristic can be attained, but a problem tends to occur in that a delay time increases.
In view of the foregoing, the use has been made of a low-pass filter in which an induction m-type is combined with a window trap for providing a phase characteristic and a steep cut-off characteristic. However, the use of such low-pass filter tends to increase a delay time undesirably.
Thus, since the respective bands of the I and Q signals are restricted to 1.5 and 0.5 MHz, respectively, by means of the low-pass filter, the Q and I signals tend to be delayed about 2.35 .mu.sec. and about 0.18 .mu.sec., respectively, as compared with the brightness signal Y.
In order to compensate for those delay times, delay lines are employed to compensate for a difference between the delay time of the Q signal and that of any one of the brightness signal and the I signal, so as to thereby equalize the delay times of the brightness signal Y, the I signal and the Q signal.
The delay time added to the brightness signal Y and the delay time added to the I signal are 1.4 .mu.sec. and about 1.2 .mu.sec., respectively.
It is, however, to be noted that the brightness signal Y as well has its band restricted by a low-pass filter to 6 MHz as a transmission bandwidth. Therefore, if a delay time attributable to this low-pass filter and any other processing such as an aperture correction is added, the delay time thereof can be subtracted, but the brightness signal Y is generally given a delay time of about 1 .mu.sec. (See, "NHK Terebi Gijutsu Kyokasho (Jou)" (NHK TV Technical Text), pages 17 to 19, published by Nippon Hoso Kyoukai, Apr. 10, 1989.)
The I and Q signals to which the respective delay times have been added in the manner described above are supplied to a quadrature two-phase amplitude modulator in which they are combined together with no cross-talk to provide a single color signal. This color signal is subsequently added to the brightness signal having the delay time adjusted by means of the delay line, so as to thereby provide an NTSC output signal.
Since the circuitry so constructed as described hereinabove is an analog circuit, the delay line to be inserted in each of the brightness signal and the I signal is employed in the form of a delay circuit having a concentrated constant such as, for example, a resistor, a capacitor or a coil, for example, a delay circuit of induction m-type capable of exhibiting a linear phase characteristic, or a delay cable.
Accordingly, the phase characteristic and the amplitude characteristic tend to be adversely affected as a result of a change in temperature and/or a change with time and, therefore, the image quality tends to be adversely affected considerably. Where the delay cable is employed, a reflection of a high frequency occurs, thereby adversely affecting the image quality.
In order to suppress the reduction in characteristic resulting from the change with time, it is a recent trend to digitize the camera. Although the camera wherein both of a signal processing circuit and an encoder are digitized is today available (See, "Signal Processing LSI for Totally Digitalized Color Camera", Technical Report of the Society of Television, 1984 Vol. 8, No. 3, pages 27 to 32.), a digital camera wherein only the signal processing circuit is digitized to reduce power consumption while an analog encoder is employed is rather widely used and, therefore, the use of the analog delay line cannot be avoided, failing to make best use of the digital features.